Gas-Phase Chemistry of Curium:  Reactions of Cm⁺ and CmO⁺ with Alkenes, Acetonitrile, and Hexafluoropropene

Abstract

The metal ions, Cm+, U+, and Tb+, and their oxides, MO+, were reacted in the gas phase with alkenes, acetonitrile, and hexafluoropropene. Product compositions and abundances provide a survey of essential aspects of the gas-phase chemistry of the curium cation, now the heaviest element for which such systematic studies have been carried out (U and Tb were included to provide comparisons). Of particular interest is the difference in behavior between the 4f lanthanide and 5f actinide series and variations in chemistry across the actinide series. The primary emphasis was on reactions with alkenes, particularly dehydrogenation, as indicative of the ability of a M+ to activate C−H bonds (the extent of C−C activation was generally in parallel with that of C−H activation). With acetonitrile and all of the alkenes (except ethene), the three M+ ions induced dehydrogenation. Variations were evident for the different reactant substrates, but the overall qualitative ordering of dehydrogenation efficiency was U+ > Tb+ > Cm+. This order is consistent with the variation in electronic promotion energies (PE) required to provide two spin-unpaired, non-f valence electrons at the M+, i.e., a [Rg]f n -2ds1 configuration (“Rg” = Xe for the lanthanides and Rn for the actinides), to enable its insertion into a C−H bond. The reduced reactivity of Cm+ relative to U+ and Tb+ suggests that the closed-shell 7s electrons of ground Cm+ (8S [Rn]5f7s2) are ineffective in enabling C−H activation and that promotion to the 10D [Rn]5f76d17s1 configuration is prerequisite. The PE[Cm+] (48 kJ mol-1) is only slightly greater than PE[Tb+] (39 kJ mol-1) and the significantly greater reactivity of Tb+ vs Cm+ may reflect that the transition to the prepared “divalent” state (d1s1) is parity forbidden for Cm+. The three MO+ were substantially less reactive than the naked M+ but were comparably reactive to one another, consistent with a multicentered activation process, which is less efficient than direct cleavage of a C−H bond by M+ insertion. With hexafluoropropene, the primary reaction channel was F abstraction and the discrepant reactivities reflected the propensity for U to oxidize to higher valence states compared to Cm or Tb. The terminal MF n + were CmIIIF2 +, TbIIIF2 +, and UIVF4 +, and among the MO+, only UO+ induced F abstraction, producing UOF n + with n = 1−3.

Berkelium and californium organometallic ions

“…Because the ablation conditions are sufficiently energetic to produce substantial amounts of M + (ionization energy of M ≈ 600 kJ mol −1 ), a substantial population of excited state M + * should also be produced in the ablation plume. The finding of primarily ground state M + is, however, consistent with the results of metal ion -molecule reactions carried out using comparable ablation conditions [9][10][11][12][13][14]. We therefore postulate that the dominant M + /R association processes occur after partial expansion and concomitant cooling of the ablation plume.…”

“…Because significant amounts of oxide cluster ions do not generally appear upon ablation of binary actinide oxides [9][10][11][12][13][14], it is presumed that the clusters observed in the present study resulted from coalescence in the dense ablation plume by reactions such as: AnO + + AnO → An 2 O 2 + . The Cf 2 O + may have resulted from combination of Cf + and CfO, consistent with the tendency of Cf to exhibit reduced oxidation states and the dearth of CfO + .…”

“…A "monovalent" An + can readily form a single covalent bond, and a "divalent" An + two such bonds. For the gas-phase transuranium actinide ions, it has been established that the quasi-valence 5 f electrons are sufficiently localized and energetically "anchored" to the closed-core Rn nuclear/electronic assembly ([Rn]) that they do not actively participate in organometallic bonding [10][11][12][13][14]. The ground state electronic configurations of the free An + ions, Np + through Cf + , are [Rn]5 f n−1 7s 1 , with an exception being Cm + .…”

“…These reactions typically produce organoactinide complex ions via elimination of stable molecules such as H 2 [9][10][11][12][13][14]. The dominant products are generally well-represented as an An + electrostatically bonded to one or more unsaturated organic ligands via π-orbital interactions; none of the products were considered to involve direct σ-type bonding between the actinide and carbon.…”

Metal ion - molecule reactions of Bk+: comparison with Pu+ and Tb+

“…In recent years, we have applied a different experimental approach for studying gas-phase actinide ion chemistry, laser ablation with prompt reaction and detection (LAPRD), which is particularly well-suited to studying heavier actinides. Previous reports have described reactions of Th + , U + [9], Np + , Pu + [10], Am + [11], Cm + [12] and Cf + [13] with a variety of gaseous reagents, with an emphasis on interactions with simple organic molecules and the resulting gas-phase organoactinide chemistry. Element z = 98, Cf, was investigated by LAPRD prior to the preceding element z = 97, Bk, because the most readily available Bk isotope, 249 Bk, β-decays to 249 Cf with a sufficiently short half-life (t 1/2 = 320 days) that ∼ 1% had transmuted to the 249 Cf daughter by the time the experiments were carried out.…”